An example of a publication of the conventional technology of an ignition control system for an internal combustion engine includes Japanese Patent Laid-Open Publication No. 2000-9007. In this publication, a technology for preventing knocking and acceleration shock and the like in an internal combustion engine through controlling ignition timing at the time of rapid acceleration of the internal combustion engine is disclosed. At the time of rapid acceleration (when a throttle opening of a throttle valve is rapidly shifted to the opening direction from a state in which the throttle opening is kept constant), the ignition timing is retarded to prevent the shock caused by the rapid acceleration.
Generally, in an independent intake type internal combustion engine, base ignition timing is determined based on a throttle opening and an engine rotation speed, and the base ignition timing is corrected based on various parameters such as a cooling water temperature, atmospheric pressure, and a gear position of a transmission to evaluate the final ignition timing.
However, in the internal combustion engine described above, when a sequence of operations in which a throttle valve is rapidly opened from a predetermined throttle opening and then rapidly closed is started and finished within a very short period of time (a period of time shorter than the period of one cycle of the internal combustion engine), the internal combustion engine continues operation as if the variation of the throttle opening was not caused by the rapid opening of the throttle valve. Therefore, the actual ignition timing is largely different from the optimal ignition timing, which is required by the internal combustion engine, thereby disadvantageously causing knocking or the like.